There are known a number of photosensitive elements having a photosensitive layer on a support wherein images are formed by imagewise exposure. Among these, a technique of forming images through heat development is known as a system capable of simplifying image forming means and contributing to the environmental protection.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the photomechanical process field to reduce the quantity of spent solution. Needed in this regard is a technology relating to photothermographic elements for use in photomechanical process which can be effectively exposed by means of laser scanners or laser image setters and produce distinct black images having a high resolution and sharpness. These photothermographic elements offer to the customer a simple thermographic system that eliminates a need for solution type chemical agents and is not detrimental to the environment.
The technology of forming images through heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic elements generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an organic binder matrix. Photothermographic elements are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the reducible silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
One of the outstanding problems associated with photothermographic elements is fog. A number of studies have been made to reduce the fog of thermographic silver halide photosensitive materials. A variety of antifoggants are disclosed in a number of patents. For example, U.S. Pat. No. 3,589,903 discloses mercury salts; U.S. Pat. No. 4,152,160 discloses carboxylic acids such as benzoic acid and phthalic acid; U.S. Pat. No. 4,784,939 discloses benzoylbenzoic acids; U.S. Pat. No. 4,569,906 discloses indane and tetraphosphorus carboxylic acids; U.S. Pat. No. 4,820,617 discloses dicarboxylic acids; U.S. Pat. No. 4,626,500 discloses hetero-aromatic carboxylic acids; U.S. Pat. Nos. 4,546,075, 4,756,999, 4,452,885, 3,874,946, and 3,955,982 disclose halogenated compounds; U.S. Pat. No. 5,028,523 discloses halogen molecules or halogen atoms associated with heterocyclic rings; U.S. Pat. No. 4,103,312 and GB 1,502,670 discloses palladium compounds; U.S. Pat. No. 4,128,428 discloses iron group metals; U.S. Pat. Nos. 4,123,374, 4,129,557 and 4,125,430 disclose substituted triazoles; U.S. Pat. Nos. 4,213,784, 4,245,033 and JP-A 26019/1976 disclose sulfur compounds; U.S. Pat. No. 4,002,479 discloses thiouracil compounds, JP-A 123331/1975 discloses sulfinic acids; U.S. Pat. Nos. 4,125,403, 4,152,160, and 4,307,187 disclose metal salts of thiosulfonic acids; JP-A 20923/1978 and 19825/1978 disclose mixtures of a metal salt of thiosulfonic acid and sulfinic acid; JP-B 50810/1987, JP-A 209797/1995 and 43760/1997 disclose thiosulfonates; and JP-A 42529/1976 and JP-B 37368/1988 disclose disulfides.
These compounds, however, have drawbacks. Some are unsatisfactory in antifogging ability, some cause a drop of maximum density (Dmax) when added in large amounts, and some exacerbate image storage stability after development. There is a desire to have a novel antifoggant.
Additionally, JP-A 26019/1976, 207244/1982, 207140/1985 and 251838/1990, U.S. Pat. Nos. 2,910,377 and 3,074,809 disclose salicylic acid and derivatives thereof, but refer nowhere to their effect in ultrahigh contrast dry silver systems.